This is the Mod-1 21T Pinion Gear from Castle Creations. FEATURES: Steel construction Designed for 1/8 scale cars INCLUDES: One Mod-1 21T Pinion Gear One 4x4mm Set Screws SPECS: Length: 12mm (0.47") Outer Diameter: 23mm (0.91") Inner Diameter: 5mm (0.20") Castle CreationsPart CSE010-0065-12